DRILL-2961: Part 1--Prep., Hyg.: Split DrillStatement -> DrillStatement{,Impl}.
Preparation for query/network timeout methods:
- Moved class ...jdbc.DrillStatement to ...jdbc.impl.DrillStatementImpl.
- Created interface ...jdbc.DrillStatement interface, extending
java.sql.Statement (for place for Drill-specific narrowed declarations and
documentation). Declared DrillStatementImpl to implement new DrillStatement.
- Updated references.
- Adjusted visibility (temporarily make somethings publich until DRILL-2089 is
done more).
Misc. code hygiene:
- Purged extraneous imports of ResultSet.
- Purged TODO made obsolete by UserException.
- Misc. whitespace.
